Question About Big Block Heads

[Monte M.]

I bought my 72, with a 454 in it, about twenty five years ago. It came with the stock lower end, but the heads were the large port closed chamber heads. It had the GM LS-6, LS-7, single four barrel intake and carb on it.

I was told the cam is the 1970 365 horse cam. What I am getting at is, I know the large port stuff will really come into play at higher RPM than that engine will ever see, but what is the closed chamber doing for the engine.

I know this was not a well thought out engine.

I think he thought the closed chamber would up the compression enough to make it really run. It actually does alright.

The guy was smart enough to keep all the original parts that he removed. I got them when I bought the car. I am now putting the car back to stock looking. Smog pump and all.

THE QUESTION: Do any of you think leaving the closed chamber heads on the engine will be a benefit. I am putting the stock oval port intake back on the engine along with the original exaust manifolds.

I plan on buying a special grind cam to make it sound pretty and maybe even run a little better. I still want to run pump gas, but want it to run well.

I am replacing the pistons, but I would have to raise them quite a bit to get the original heads to get that motor going.

I am open to suggestion with using some or none of these parts.

Thanks for taking the time

[Joe R.]

Re: Question About Big Block Heads

I bought my 72, with a 454 in it, about twenty five years ago. It came with the stock lower end, but the heads were the large port closed chamber heads. It had the GM LS-6, LS-7, single four barrel intake and carb on it.

I was told the cam is the 1970 365 horse cam. What I am getting at is, I know the large port stuff will really come into play at higher RPM than that engine will ever see, but what is the closed chamber doing for the engine.

I know this was not a well thought out engine.

I think he thought the closed chamber would up the compression enough to make it really run. It actually does alright.

The guy was smart enough to keep all the original parts that he removed. I got them when I bought the car. I am now putting the car back to stock looking. Smog pump and all.

THE QUESTION: Do any of you think leaving the closed chamber heads on the engine will be a benefit. I am putting the stock oval port intake back on the engine along with the original exaust manifolds.

I plan on buying a special grind cam to make it sound pretty and maybe even run a little better. I still want to run pump gas, but want it to run well.

I am replacing the pistons, but I would have to raise them quite a bit to get the original heads to get that motor going.

I am open to suggestion with using some or none of these parts.

Thanks for taking the time

Monte,

I don't know the specifics on your engine, but these are well-engineered cars. In other words, if you put the engine back to stock, I bet it will run better than it does now. If you want some extra ponies and RPM, port the heads. I would recommend using the Speed Pro cam that is ground to factory specs. You can get it at NAPA.

Joe

[Norris W.]

Re: Question About Big Block Heads

I've gotta' disagree on the "well engineered" and runnin' better with the changes than a stock '72 would have. The new compression drop turned what was formerly a good performer into a slug. I took a stock '71 LS5 Corvette, installed closed chamber oval port heads from a '70, and called Crane and got a cam upgrade. What I asked for was something that would have a smooth idle as per a stock LS5, but all they could do performance wise with the smooth idle limit. I don't recall the grind, but the car was an animal. It would haze the tires from a roll in first gear and break 'em loose with a hard shift to 2nd, which is fairly unusual for a Corvette.

Unless you need to freshen the rotating assembly, I think you'd be happy with a set of closed chamber oval port heads and the cam upgrade you're considering. Actually the rectangle port heads probably don't hurt the engine as much down low as going back to the lower compression and oval ports of the stock heads would. I think you need to clearly determine your priorities. I don't think leaving the rectangle port heads on with an oval port intake is a good idea. It works with some combinations, but with others gasket seal can be an issue. I'm not sure if that's the case with a stock '72 intake, but a knowledgeable person will be able to identify the heads as being incorrect at a glance, so there goes the stock look. If you really are after the stock look, I'd probably put the stock heads and intake back on and buy pistons that will get your compression ratio up in the 10's where it would've been in 1970 before the gov't shot performance between the eyes. Add the cam upgrade you're considering and you'll be in business.

Incidentally, the LS6 & LS7 intakes are not the same. The LS6 was a low rise intake and by most estimates a performance killer to the tune of about 30 HP. The LS7 was the conventional GM high rise intake, similar to those used on the SHP 396's and 427's from '65 up through '69.

[Joe R.]

Re: Question About Big Block Heads

I have read on here that the early BB distributors have the best curves from a performance standpoint. Perhaps some ponies can be gained here, but maybe only with the smog equipment disabled.

Joe

[Duke W.]

Re: Question About Big Block Heads

If you want to maximize power without screwing up the idle and low end torque, the first thing to do is massage the original oval port heads. If they are small chamber that's fine.

Use the OE cam (same as earlier L-36/68), but with the longer stroke 454 you have an option to install it 5 degs retarded from the OE indexing if you want to trade a little low end torque for a little more top end power.

Choose pistons and head gaskets using actual measured deck clearances and chamber volume to achieve between 10 and 10.25:1 true compression ratio. (See the Fall 2009 Corvette Restorer article on this subject.)

The dist. should be set up with a 12" VAC connected to full manifold vacuum. Set up the centrifugal as aggresive as possible with total WOT spark advance of 38-40 degrees.

If you're interested in a custom cam that can do better, talk to me, but I can't design a cam without accurate head flow data. I think this engine will respond to the same design philosoply I used to design the McCagh Special camshaft, which equaled or exceeded expectations.

These hydraulic lifter big blocks are torque monsters that can be made to produce more power below about 4500 than a SHP big block, and that puts the power in a rev range that you can use most of the time, just like The Special 300 HP small block configuration that makes better low end power than a OE L-79 with equal top end power.

Duke